The present invention relates to a variable gain amplifier (VGA) and a driver implementing the VGA for driving an optical device.
Recently, advanced algorithms have been requested in the optical communication system as the transmission speed has become faster. New algorithms are, for instance, the amplitude modulation by multi-levels, the phase modulation, and a combination of these two modulations, which is sometimes called as the quadrature amplitude modulation (QAM) by 22n (n=2, 3 . . . ) levels, namely, 16 QAM, 64 QAM and so on.
In the multi-level modulation, an apparatus is requested to amplify a signal generated by, for instance, a digital-to-analog converter (DAC) to an enough amplitude for driving an optical device linearly. That is, the apparatus implemented within the QAM system is necessary to have an enough dynamic range. An optical transmitter in the QAM system is often designed specific to the QAM system. For instance, one type of a driver with a function of a variable gain is often used in the QAM system because such a variable gain amplifier (VGA) may amplify a signal with a wide dynamic range in the amplitude thereof to a preset amplitude (a saturated amplitude) as keeping a linearity thereof. A prior document by Mayer et. al in IEEE, JSSC 26(11) (1991) has disclosed one type of a VGA. While, United States Patents, U.S. Pat. No. 7,076,226 and U.S. Pat. No. 5,079,515, have disclosed a technique to enhance the linearity of a VGA.
The VGA disclosed in the IEEE document increases an operating current to enhance the linearity thereof, which directly results in the increase in the power consumption. A VGA disclosed in U.S. Pat. No. 7,076,226 has increased the linearity thereof by cancelling a portion of a trans-conductance of an active device, which is equivalent to reduce a gain thereof. Accordingly, in order to attain a preset gain, two or more stages of the amplifiers are necessary to be installed, which means that the power consumption unavoidably increases. A VGA disclosed in another patent document, U.S. Pat. No. 5,079,515, requires for attaining an enhanced linearity that an input signal has preset amplitude.